The present invention relates to a diffusion plate of a direct-type backlight module, especially to a diffusion plate that improves efficiency of light diffusion effect and enhances brightness.
Due to excellent display quality and low cost, the CRT monopolizes the market for quite a long time. However, for environmental protection as well as power saving, the CRT got a lot of problems and can't match requirements of compact size, light weight, and low power. Thus Thin Film Transistor Liquid Crystal Display (TFT-LCD) with features of high definition, space-saving, low power and no radiation has become mainstream. Inside the liquid crystal display, the Liquid crystal panel doesn't illuminate. Instead, a back light module is disposed under the liquid crystal panel as a light source.
The prosperous development of the TFT-LCD technology causes invention and demand of liquid crystal displays. The difference between the LCD TV and LCD monitor is in size, brightness and viewing angle. In order to meet requirements of large size and high brightness, the conventional edge-light backlight module is replaced by direct-type backlight module. Besides different numbers of lamps, a light guide panel is changed into a diffusion plate.
Refer to FIG. 1, a common direct-type backlight module consists of a refractor 10′, a plurality of light sources 11′, a light diffusion plate 12′, a lower diffuser film 13′, a prism sheet 14′ and an upper diffuser film 15′. As to backlight module, there are five common ways for brightness enhancement. Firstly, two pieces of horizontal or vertical prism sheets are used. By refraction index and angle of the prisms, the incident light is refracted, passing through the serrated member for control of the refraction angle so as to collect the light already diffused, reduce light loss and enhance brightness. But on LCD TV, only horizontal prisms are applied for collecting light due to limits of horizontal view angle. Secondly, a multi-layer interference filter is applied. A multilayer film with feature of birefringence is made from two polymers with different refraction index by co-extrusion then stretching and lamination then stretching. The polarized light perpendicular to the interference filter is transmitted through directly while the polarized light parallel to the filter is reflected to the backlight and recycled. Thus more light is transmitted through he LCD and the luminance is increased. The film works through polarization recycling. It recycles most of the light that is normally lost in the rear polarizer. For example, a dual-brightness enhancement film (D-BEF) can be used. The third, a cholesteric reflective polarizer is used. The fourth, a fluorescent brightening layer that converts UV light from Cold Cathode Fluorescent Lamps into visible light is coated on the diffuser plate. The fifth, increase current or numbers of the Cold Cathode Fluorescent Lamp (or other light source). No matter increase lamps or add brightness enhancement film on the backlight module for brightness enhancement, the cost will increase. Take a backlight module of 32-inch LCD-TV as an example, key components—prism sheet accounts for about 38%, lamps 20%, diffusion sheet 4%, reflector 4% and diffuser plate 10%. Once the use of optical films is reduced, especially the prism sheet, the cost can be reduced.
Moreover, the integration of the diffusion plate is always concerns of backlight module manufactures and optical sheets producers. The related products can directly reduce cost of backlight modules and has great influence on manufacturing processes of the backlight modules.